For certain applications, it is desirable to have a magnetic loop sensor, tuned by the addition of capacitance to resonate in the frequency range of 1-1000 MHz. In order to detect very weak magnetic fields such a sensor must generate extremely low levels of noise and consequently must have extremely low resistance and hence low loss. The lowest loss sensors are made of superconductor materials. Until recently, all superconductor materials had to be cooled below 30 K. to operate as superconductors. This requirement added significantly to the cost and complexity of systems which relied on these materials. The sensor design described here is one appropriate to high temperature superconductors, i.e., superconductor materials whose superconducting transition temperature (critical temperature or T.sub.c) is higher than 30 K. This latter class of materials, also known as cuprates, oxide superconductors and perovskites, is better suited to use in thin film form than in bulk forms. This physico-chemical difference necessitates new device and circuit designs to make these materials useful in superconducting applications. The design can also be fabricated using conventional superconductors, like niobium, which are available in thin film form.
For certain magnetic resonance imaging (MRI) applications, resonant frequencies of approximately 5 MHz and sufficiently low resistance that the coil has a resonant quality factor (Q) of not less than 10.sup.4, and even as high as 10.sup.6, are desired. The low resistances implied by these high values of Q ensure that the coil's internally generated thermal noise will be less than the noise generated by other noise sources within the imaging system, such as the tissue or object being imaged or the preamplifier coupled to the coil. To achieve such high Q, it is necessary that the equivalent series resistance of the LC resonator be less that approximately 100 .mu..OMEGA. to 1 m.OMEGA.. Such low resistance is achieved by the use of superconducting thin-film metallization in both the coil and the capacitor. A key advantage of this approach is that the sensor can be produced with a single superconductive film, and as a result it is more easily and reproducibly manufactured.
Several embodiments of this invention are disclosed herein. One embodiment of this invention consists of a multi-turn spiral coil (having inductance L) with an internal distributed interdigital capacitor (having capacitance C). The device operates in a self-resonant mode. One variation of this design has no connection between the inner end of the inductor coil and its outer end. Such a configuration is possible to fabricate in a single layer of superconductor with no crossover, that is, with no intervening dielectric layer.
An alternative design employs a single turn of interdigitated capacitor and has a crossover connection between the inner end of the inductor spiral and its outer end. This embodiment consists of a multi-turn spiral coil (having inductance L) connected to a surrounding interdigital capacitor (having capacitance C). It is useful for certain magnetic-resonance-imaging (MRI) applications, employing resonant frequencies of approximately 5 MHz, having an inductance of the order of 1 .mu.H, and an equivalent series of 1 m.OMEGA. or less. The corresponding resonant quality factors (Qs) are 10.sup.4 to 10.sup.6. Such low loss is achieved by the use of superconducting thin-film metallization in both the coil and the capacitor. Even the crossover (which connects to the inside end of the spiral coil and crosses over the other turns of the coil to reach one terminal of the capacitor) must be superconducting, which is achieved by the use of two thin films of superconductor with an intervening layer of insulating thin film. This design has a higher effective capacitance than does the single layer variant, which results in a lower resonant frequency. Because it does not require interdigital capacitors throughout the circuit, it can also be made with higher inductance than the single layer variant, allowing it to operate at lower frequencies at the expense of a crossover.
Other embodiments of the present invention employ two multi-turn spiral coils, placed in proximity to each other and with a sense of current flow such that their mutual inductance enhances their self inductances, coupled together by two annular capacitors at their inner and outer extremities in order to form a resonant circuit. Here the capacitance of the structure is increased in order to reduce the operating frequency. In these embodiments a key advantage is that the dielectric of the annular capacitors can be any suitable layer of material and can be made arbitrarily thin in order to reduce the resonant frequency.